A conventional key opening apparatus is described hereinafter with reference to FIG. 5. In FIG. 5, operator 3 is away from car 1 having door 2. In this case, it may take some time from depressing a button on key 4 before opening door 2.
On the other hand, operator 5 near to car 1 desires door 2 to open by just touching knob 7 of door 2, and key 8 satisfying this request was already developed. A use of this key 8 saves operator 5 from taking the trouble to put down large baggage on the ground before depressing a button on key 8, and allows the operator 5 to release the key of the door by just touching knob 7. In this case, the radio wave 9 may travel within a short range; however, the radio wave preferably travels fast. On the contrary, operator 3 away from car 1 wants radio wave 10 to travel in a longer range, but it may travel at a slower speed.
A key opening apparatus, which communicates with key 4 delivering a radio wave of slower transmission speed, comprises the following elements as shown in FIG. 6: (This key-opening apparatus 11 is mounted in car 1.)                (a) antenna 15 for receiving radio wave 10 including a signal of a slower transmission speed (1 kbps);        (b) receiving section 16 for receiving the signal received by antenna 15;        (c) filter 17 for passing the signal of 1 kbps supplied from receiving section 16;        (d) comparator 18 for receiving an output from filter 17;        (e) decoder 19 for decoding an output from comparator 18; and        (f) output terminal 20 for receiving an output from decoder 19.        
In this key-opening apparatus structured above, radio wave 10 is supplied to antenna 15, and converted its frequency by receiving section 16, then filter 17 passes only a signal having a transmission speed of 1 kbps. This signal is shaped its waveform by comparator 18 before entering into decoder 19, which decodes the signal. When decoder 18 tells that door 2 may be opened, output terminal 20 outputs a key-opening signal to release the key of door 2.
Another key-opening apparatus that communicates with key 8 delivering a radio wave of higher transmission speed comprises the following elements as shown in FIG. 7: (This key-opening apparatus 21 is also mounted in car 1.)                (a) antenna 22 for receiving radio wave 9 including a signal of a higher transmission speed (15 kbps);        (b) receiving section 23 for receiving the signal received by antenna 22;        (c) filter 24 for passing the signal of 15 kbps supplied from receiving section 23;        (d) comparator 25 for receiving an output from filter 24;        (e) decoder 26 for decoding an output from comparator 25; and        (f) output terminal 27 for receiving an output from decoder 26.        
In this key-opening apparatus structured above, radio wave 9 is supplied to antenna 22, and converted its frequency by receiving section 23, then filter 24 passes only a signal having a transmission speed of 15 kbps. This signal is shaped its waveform by comparator 25 before entering into decoder 26, which decodes the signal. When decoder 26 tells that door 2 may be opened, output terminal 27 outputs a key-opening signal to release the key of door 2.
Conventional key-opening apparatuses 11, 21 discussed above are used in pairs, which require a large space and consume a great power.